In recent years, the micronization of pattern formation by lithography has been rapidly advanced in the manufacturing process of a semiconductor element, a liquid crystal element, or the like. As the technique of micronization, there is a method to shorten the wavelength of irradiation light.
Recently, KrF excimer laser (wavelength: 248 nm) lithographic technique is introduced, and ArF excimer laser (wavelength: 193 nm) lithographic technique and EUV (wavelength: 13.5 nm) lithographic technique which further shorten wavelengths, have been investigated.
As the technique for further increasing resolution, the development of a so-called negative development process in which a fine pattern can be formed without generating resist residue by the combination of a positive resist composition and a negative developing solution, or the combination of a positive resist composition, a negative developing solution, and a positive developing solution, has been advanced (Patent Document 1).
In addition, a so-called chemically amplified resist composition containing a copolymer which becomes alkali-soluble by the removal of an acid leaving group by the action of acid, and a photoacid generator has been proposed, for example, as a resist composition which can suitably cope with the shortening of the wavelength of irradiation light and the micronization of pattern, and the development and improvement thereof have been advanced.
As a copolymer for chemically amplified resist used in ArF excimer laser lithography, an acrylic copolymer which is transparent with respect to light having a wavelength of 193 nm has attracted attention.
For example, Patent Document 2 to be described below discloses a copolymer for lithography formed by using (A) a (meth)acrylic acid ester having an ester bond with an alicyclic hydrocarbon group having a lactone ring, (B) a (meth)acrylic acid ester having an ester bond with a group capable of leaving by the action of acid, and (C) a (meth)acrylic acid ester having an ester bond with a hydrocarbon group having a polar substituent or an oxygen atom-containing heterocyclic group as monomers.
However, a copolymer of (meth)acrylic acid ester is generally polymerized by radical polymerization. Generally, since the ratio of copolymerization reactivity between the monomers is different in a multi-component copolymer containing two or more kinds of monomers, the composition of monomer units (copolymerization composition) in the copolymer produced at the initial stage of polymerization is different from that in the copolymer produced at the late stage of polymerization, and thus the copolymer to be obtained has a composition distribution.
If there is variation in the composition of monomer units in a copolymer, the solubility in a solvent tends to be ununiform, and when preparing a resist composition, it takes a long time to dissolve the copolymer in a solvent or the number of manufacturing processes increases due to the generation of insoluble matters, whereby the preparation of the resist composition is hindered in some cases. In addition, the sensitivity of the resist composition to be obtained tends to be insufficient.
In order to obtain a resist having high sensitivity, for example, Patent Document 3 discloses a method for producing a copolymer for photoresist, the method including a supplying process of supplying a monomer solution and a solution containing a polymerization initiator into a polymerization reaction system, in which the range of fluctuation of the composition of the unreacted monomers present in the polymerization reaction system is small (specifically, within the range between minus 15% and plus 15%) from the start of the polymerization reaction until the completion of the supply of the monomer solution.
Patent Document 4 discloses a method in which some of a monomer having an acid leaving group among the monomers used in the production of a copolymer for resist is supplied into a reactor in advance, and then the rest of the monomer having an acid leaving group and a mixture of other monomers are feeded thereto and polymerized. In the feeding polymerization method, since, after a copolymer having a higher molecular weight is produced at the initial stage of polymerization reaction, a copolymer having a lower molecular weight is produced as the polymerization reaction proceeds, a copolymer containing a low polar acid leaving group deviatedly on the side of higher molecular weight produced in the initial stage of polymerization is obtained by allowing only a monomer having an acid leaving group to be present in the reactor in advance.